To transmit and receive a large volume of image signal or data via a communication network, the higher speed and wider bandwidth of the communication network have been carried out in an access network for connecting the subscriber to the communication network, whereby a Passive Optical Network system (hereinafter referred to as a PON) has been introduced as defined in the recommendation G.984.3 of the International Telecommunication Union (hereinafter referred to as ITU-T). The PON is a system in which an Optical Line Terminator (hereinafter referred to as an OLT) connected to an upper-level communication network and an Optical Network Unit (hereinafter referred to as an ONU) accommodating plural subscriber terminals (PCs or telephones) are connected via an optical passive network including a trunk optical fiber, an optical splitter and plural branch optical fibers. Specifically, in a communication form, a signal from the terminal (PC or the like) connected to each ONU, or an optical signal, is passed from the branch optical fiber via the optical splitter to the trunk optical fiber for optical (time division) multiplexing and sent to the OLT, and the OLT performs a communication process for the signal from each ONU and transmits the signal to the upper-level communication network, or the other ONU connected to the OLT.
The development and introduction of the PONs have been made including a system dealing with the low speed signal of 64 kbit/sec, a Broadband PON (BPON) for transmitting and receiving the ATM cell of fixed length at the maximum rate of about 600 Mbit/sec, an Ethernet PON (EPON) for transmitting and receiving the variable length packet of Ethernet (registered trademark) at the maximum rate of about 1 Gbit/sec, and a Gigabit PON (GPON) dealing with the faster signal of about 2.4 Gbit/sec, as standardized in the ITU-T recommendations G.984.3. Further, the implementation of the fast PON capable of dealing with the signal from 10 Gbit/sec to 40 Gbit/sec will be sought in the future. As means for implementing these fast PONs, the multiplexing methods for plural signals such as Time Division Multiplexing (TDM) for making the time division multiplexing, Wavelength Division Multiplexing (WDM) for making the wavelength division multiplexing, and Code Division Multiplexing (CDM) for making the code division multiplexing have been investigated. The current PON adopts the TDM, and the GPON, for example, uses different wavelengths for the uplink (from ONU to OLT) signal and the downlink (from OLT to ONU) signal, in which the communication between the OLT and each ONU is configured to allocate the communication time of signal to each ONU. Also, the conventional configuration of dealing with the fixed length signal has changed to the configuration of dealing with the burst, variable length signal (burst signal) easy to handle more various types of signals (voice, image, data and so on).
On the other hand, in a WDM method, plural waves having different wavelengths for both the uplink signal and the downlink signal are connected between the OLT and the ONU, and each ONU receives and transmits a specific wavelength to make the communication. By allocating an individual wavelength from the OLT to each ONU to make the communication, the communication band can be remarkably improved. For example, one implementation method for a WDM-PON capable of connecting a maximum of 32 ONUs is that M downlink wavelengths (one wavelength for each of uplink and downlink is allocated to each ONU for 32 wavelengths) are allocated, that is, the number of wavelengths for use in one PON is made double the maximum number of connected ONUs. Herein, in the WDM-PON capable of connecting a maximum of 32 ONUs, following the idea of the conventional TDM-PON, there is an idea that the PON is economically constructed by limiting the number of uplink wavelengths to n (32 wavelengths or less) to reduce the number of expensive optical components. At this time, the uplink signal is transmitted by making the time division multiplexing of the transmit signals from the plural ONUs, whereby it should be noticed that a ranging procedure or the dynamic bandwidth assignment as will be described later is required in the WDM-PON.
In the above form of each PON, the distance from the OLT to each ONU is different to install the ONUs in the subscriber houses scattered at various locations. That is, since the length of optical fiber (transmission distance) including the trunk optical fiber and the branch optical fiber from the OLT to each ONU varies, the transmission delay (delay amount) between each ONU and the OLT varies, whereby even if each ONU transmits the signal at different timing, there is possibility that the optical signals outputted from the ONUs may collide and interfere on the trunk optical fiber. Therefore, in each PON, the distance between the OLT and the ONU is measured, using a technique called the ranging as defined in chapter 10 of G.984.3, for example, and then the delay of output signal from each ONU is adjusted so that the signals outputted from the ONUs may not collide.
Further, using a technique called the Dynamic Bandwidth Assignment (hereinafter referred to as DBA), the OLT decides the band of signal permitted to transmit to the ONU, based on a transmission request from each ONU, and in consideration of the delay amount measured by the above ranging, specifies the transmission timing to each ONU so that the optical signals from the ONUs may not collide and interfere on the trunk optical fiber. That is, the PON is configured to operate the communication in the state where the timing of signal transmitted and received between the OLT and each ONU is managed within the system.
In transmitting and receiving the signal between the OLT and each ONU, the signal from the ONU to the OLT has a guard time for prevention of interference having a maximum of 12 bytes at the top of the signal from each ONU, a preamble for use to decide the signal identification threshold of a receiver within the OLT and extract the clock, a burst overhead byte called a delimiter for identifying the breakpoint of the received signal, and a control signal (sometimes called an overhead or header) of the PON, which are added to the data (sometimes called a payload), so that the OLT can identify and process the signal from each ONU multiplexed on the trunk optical fiber, as defined in chapter 8.3.3 of G.984.2, for example. Since each data is burst data of variable length, a header called a GEM (G-PON Encapsulation method) header for processing the variable length data is also added to the top of each data.
On the other hand, the signal from the OLT to each ONU has a frame alignment pattern for identifying the top, a PLOAM area for transmitting the monitor, maintenance and control information, and an overhead (sometimes called a header) called a grant indication area for indicating the signal transmission timing of each ONU, which are added to the time division multiplexed data addressed to each ONU, at the top of the signal transmitted from the OLT to each ONU, so that each ONU can identify and process the signal from the OLT. A GEM header for processing the variable length data is added to the data addressed to each ONU that is multiplexed, like the signal from the ONU. The OLT specifies the uplink transmission permission timing (transmission start (Start) and end (Stop)) of each ONU in bytes to each ONU, using the grant indication area. This transmission permission timing is called the grant. And if each ONU transmits the data addressed to the OLT at the permission timing, these are optically (time division) multiplexed on the optical fiber, and received by the OLT.